A conventional portable telephone set comprises a power supply in which a secondary cell is employed and a power controller for controlling power consumed by the telephone. The power supply can be connected with a charger through charging terminals to charge current into the secondary cell. When the power supply is connected to the charger, a constant current (charging current) flows into the telephone from the charger, regardless of whether the power of the telephone is turned ON or OFF.
For example, Japanese patent application laid-open No. 3-30262 discloses a charger of a portable telephone set which can function as both a charger and an external power supply to omit a part of a power supply circuit and a circuit-changing switch in the telephone set.
However, in the conventional telephone, if the level of the charging current is set to match the current drawn by the telephone set when it is turned ON, the charging current may exceed a maximum value and overcharge the cell when the telephone is turned OFF.
On the contrary, if the level of the charging current is set to correspond to the current drawn by the telephone set when it is turned OFF, the charging current may be below the required minimum current value required for charging when the telephone is turned ON.